66 CONCHOLOGY. 
covered with epidermis, and flexed anteriorly ; hinge with two 
lamellar teeth transversely striated, one distant, posterior and 
shortened, the other anterior, long, and lateral; ligament ex- 
terior. Habitation unknown. One species. 
Castalia ambigua. 
FAMILY XIil. 
Narapea. Four genera. 
oGenue iinin. sel WiLL: 
Animal. See Castalia. 
Shell. Usually very thick, nacred within, covered with epi- 
dermis, corroded at the summits, which are dorsal and suban- 
terior; dorsal hinge formed by a double precardinal tooth, more 
or less compressed, irregularly dentated on the left valve, and 
simple on the right, together with a long Jamellous tooth under 
the ligament: ligament external, dorsal, and postapicial; two 
muscular impressions, well marked, besides those of the retractils 
muscles. The species of this genus grow more numerous daily ; 
they are found in all countries, but particularly in North Ame- 
rica. One hundred and sixty-seven defined species. Numerous 
fossil. 
Unio Batayus. Unio crassidens. ase aa 
U. crassissimus. U. obliquus. aT 
U. elongatus. U. plicatus. * ceceuun cell 
U. litoralis. U. purpuratus. A 
U, pictorum. U. radiatus. p} 
U. platyrhonchus. U. rectus. 
U. marginalis. U. retusus. 
U. tigris. U. rotundatus. 
U. Egyptiacus. U. Australis. 
U. Niloticus. .U. Nova Hollandica. 
U. Leaii. 
The following species are American : 
Unio Bengalensis.- Lea. U. lamellatus. Lea. 
U. bilineatus. o U. Morchisonianus, _,, 
